The present invention relates to a method of joining Y-based oxide superconductors. The joined material of Y-based superconductors to be made according to the invention is used for magnetic shield, superconductive magnetic carrier equipment, superconductive permanent magnet or the like.
It has been known that the melt-grown Y-based oxide superconductors exhibit exceptionally higher critical current density over the material made by sintering process. A new preparing method named MPMG method (e.g. M. Murakami et al, Jpn. J. Appl. Phys. 28 (1989) 1189)) made it possible to disperse fine normal conductive phase into the crystals. The producted superconductors exhibit high critical current density even in a high magnetic field. Using such materials, applications to magnetic shield etc. are being investigated.
For applying the Y-based oxide superconductors to magnetic shield, it is required to enlarge the area of a piece of superconductor (bulk). It is desirable to put the crystal orientation of bulk in order, because the shielding current flows in the plane spanned by the a-axis and the b-axis of crystals. Consequently, the crystal orientation (the c-axis) is set parallel to the magnetic field. The preparing methods of bulk involving aligned crystals are investigated in many ways, but, at the present time, it is all one can do to prepare crystals as large as several centimeters at most.
For a technique in making a large magnetic shield plate, laminating bulks where the joints not to come to the same position was proposed, but there arose a problem to cause the leakage of magnetic field in a strong magnetic field (e.g. 48th Low Temperature Technology Superconductance Society Proceedings, A3-10 A3-11).
As a means to solve these matters, it was considered to make the bulks large by joining the bulks with preferable crystal orientation. K. Salama and V. Selvamanickam (Appl. Phys. Lett, 60 (1992) 898) and others joined the two pieces of YBa.sub.2 Cu.sub.3 O.sub.7-.delta. superconductor (Y123) without deterioration of characteristics at the joined interface by heating them up to a temperature range from 910.degree. C. to 930.degree. C. and keeping for 30 hours under a uniaxial pressure range from 2 to 6 MPa, where nearly pure Y123 bulks were used. For practical use, Y123 bulks with large amount of fine normal conductive phase Y.sub.2 BaCuO.sub.5 (Y211) are recommended, because they allow high critical current density even in a strong magnetic field.
When we attempted to join the recommended bulks by conventional method, it is difficult to make the joined interface that retains the characteristics of original bulks. The joining process is performed usually through the processes of high-temperature creep, diffusion and rearrangement of atoms. With Y-based oxide superconductor with much Y211 phase dispersed, however, the creep transformation is hard to occur because of composite material comprising Y123 mother phase and Y211 dispersed phase. As a result, portions such as voids are liable to remain at the joined interface. In contrast, such heat treatment for a long time at high temperature near the melting point (about 1000.degree. C.) of Y123 mother phase promotes the deformation of Y123 bulk and the flow-out of liquid phase and leaves the second phases such as Y211 and CuO behind at the interface, resulting in a cause of deterioration of characteristics at the joined interface.
The invention originates in view of such situation and has a purpose to perform the joining of Y-based oxide superconductors to be offered to practical use.